Disturbances which occur in a data channel can be categorized as either additive or multiplicative. An undesirable additive disturbance signal is simply added to the information (data) signal. An undesirable multiplicative disturbance or data density change causes a modulation of the data signal.
In a data channel where the signal sensing transducers are magneto-resistive (MR) sensors exposed to the air in the air bearing surface of a slider assembly and a rotating magnetic disk, additive disturbances can occur due to physical frictional contact of the sensor(s) with the moving recording surface of the disk. The disturbances result from the friction-generated elevated temperature (up to 120.degree. C) at the contact spot. This produces a small yet sudden increase in temperature of the MR sensor; e.g., in the order of 1.degree. C averaged over the entire sensor within about 50 to 100 nanoseconds. Due to the nonzero temperature coefficient of resistance of the MR sensor (approximately 0.003/.degree.C for permalloy), the sensor resistance will increase with this sudden temperature rise. The heat conducted into the MR sensor from the hot spot will diffuse slowly to the environment of the sensor, causing the resistance increase to decay slowly to the original value. Typically, a drop to about 30% of the thermally induced resistance change will occur in 1.5 to 5 .mu.s. The MR sensor is used for detecting magnetic signals by the magneto-resistive effect. The sensor is biased with a constant direct (DC) current to convert the resistance changes due to the magnetic information into a data voltage signal for later amplification. The thermally induced resistance change will then lead to an additive disturbance upon which the data signal is superimposed. The nonlinearity of such an MR sensor increases with increasing magnetic signal excursions around its bias point. Therefore, these magnetic excursions are kept sufficiently small giving at most a relative change in sensor resistance of + 0.3%. Therefore thermal disturbance signals can be up to four times the base-to-peak data amplitude and possibly even greater.
Such a combination of signal and disturbance causes many problems with signal detection in the data channel. The automatic gain control (AGC) circuit in the channel may fade out quickly during the transient and recover only slowly. Even if the AGC circuit were to accommodate the disturbed signal, the thermal transient would still result in a peak shift; i.e., the data signal is differentiated for peak detection; and as a result of this the thermal transient will also be differentiated. This leads to an extra zero crossing and a shift of the zero-crossing level directly after the thermal transient.
There is a need for a method and apparatus for suppressing additive transient disturbances that are caused in a data channel by a temperature change (hereinafter referred to as a thermal asperity) in an MR sensor due to frictional contact with the moving recording surface of a magnetic disk. Such suppression cannot be achieved by the prior art approaches known to applicants because the MR sensor as used in hard disk products is not provided with a center tap (which could be utilized to balance out thermal transients), the spectral content of the thermal transient disturbance is too close to the spectral content of the data signal to be filtered out, and the problems associated with high-frequency AC biasing are for all practical purposes unsolvable.
In addition to the prior art approaches above described, U.S. Pat. 3,566,281 constitutes the most pertinent prior patent art of which applicants are aware. This patent discloses positive and negative peak detectors which are offset by a constant voltage and averaged and subtracted from a delayed input signal. Note that the outputs of each detector are cross connected for resetting each detector to zero after the pulse has been processed. This patent merely describes clipping level circuitry in which, as the positive peak detector detects a positive peak, the negative peak detector is reset to zero, and vice versa. This patent and others (such as U.S. Pat. Nos. 3,473,131 and 4,356,389) less pertinent do not and cannot solve the problem to which applicants' invention is directed. They do not disclose means for insuring that both detectors respond rapidly to the onset of an additive disturbance, or means to reduce residue components and produce an output signal free of the additive disturbance.